A Review of Terminology Used to Describe Soot Formation and Evolution under Combustion and Pyrolytic Conditions.

This review presents a glossary and review of terminology used to describe the chemical and physical processes involved in soot formation and evolution and is intended to aid in communication within the field and across disciplines. There are large gaps in our understanding of soot formation and evolution and inconsistencies in the language used to describe the associated mechanisms. These inconsistencies lead to confusion within the field and hinder progress in addressing the gaps in our understanding. This review provides a list of definitions of terms and presents a description of their historical usage. It also addresses the inconsistencies in the use of terminology in order to dispel confusion and facilitate the advancement of our understanding of soot chemistry and particle characteristics. The intended audience includes senior and junior members of the soot, black carbon, brown carbon, and carbon black scientific communities, researchers new to the field, and scientists and engineers in associated fields with an interest in carbonaceous material production via high-temperature hydrocarbon chemistry.

New seeding methodology for gas concentration measurements.

This paper presents the first demonstration of the pulsed laser ablation technique to seed a laminar non-reacting gaseous jet at atmospheric pressure. The focused, second harmonic from a pulsed Nd : YAG laser is used to ablate a neutral indium rod at atmospheric pressure and temperature. The ablation products generated with the new seeding method are used to seed the jet, as a marker of the scalar field. The neutral indium atoms so generated are found to be stable and survive a convection time of the order of tens of seconds before entering the interrogation region. The measurements of planar laser-induced fluorescence (PLIF) with indium and laser nephelometry measurements with the ablation products are both reported. The resulting average and root mean square (RMS) of the measurements are found to agree reasonably well although some differences are found. The results show that the pulsed laser ablation method has potential to provide scalar measurement for mixing studies.

Sodium and potassium released from burning particles of brown coal and pine wood in a laminar premixed methane flame using quantitative laser-induced breakdown spectroscopy.

A quantitative point measurement of total sodium ([Na](total)) and potassium ([K](total)) in the plume of a burning particle of Australian Loy Yang brown coal (23 ± 3 mg) and of pine wood pellets (63 ± 3 mg) was performed using laser-induced breakdown spectroscopy (LIBS) in a laminar premixed methane flame at equivalence ratios ( U ) of 1.149 and 1.336. Calibration was performed using atomic sodium or potassium generated by evaporation of droplets of sodium sulfite (Na(2)SO(3)) or potassium sulfate (K(2)SO(4)) solutions seeded into the flame. The calibration compensated for the absorption by atomic alkalis in the seeded flame, which is significant at high concentrations of solution. This allowed quantitative measurements of sodium (Na) and potassium (K) released into the flame during the three phases of combustion, namely devolatilization, char, and ash cooking. The [Na](total) in the plume released from the combustion of pine wood pellets during the devolatilization was found to reach up to 13 ppm. The maximum concentration of total sodium ([Na](max)M(total)) and potassium ([K](max)(total)) released during the char phase of burning coal particles for φ = 1.149 was found to be 9.27 and 5.90 ppm, respectively. The [Na](max)(total) and [K](max)(total) released during the char phase of burning wood particles for φ = 1.149 was found to be 15.1 and 45.3 ppm, respectively. For the case of φ = 1.336, the [Na](max)(total) and [K](max)(total) were found to be 13.9 and 6.67 ppm during the char phase from burning coal particles, respectively, and 21.1 and 39.7 ppm, respectively, from burning wood particles. The concentration of alkali species was higher during the ash phase. The limit of detection (LOD) of sodium and potassium with LIBS in the present arrangement was estimated to be 29 and 72 ppb, respectively.

Solvent effects on two-line atomic fluorescence of indium.

We aim to investigate the potential of four different organic solvents, namely, acetone, ethanol, methanol, and isopropanol, and the organic-solvent-water mixtures as a seeding medium for the two-line atomic fluorescence technique. Water is used as the reference case. Indium, which has been previously shown to have suitable spectroscopic attributes, is chosen as the thermometry species in the present study. Acetone and methanol are shown to enhance the fluorescence signal intensity the most (approximately threefold to fivefold at stoichiometric conditions) when used. Acetone and methanol are shown to improve the fluorescence emission over the entire stoichiometric envelope of flame, most significantly in the rich combustion region, as well as a twofold enhancement in the signal-to-noise ratio.

Instantaneous temperature imaging of diffusion flames using two-line atomic fluorescence.

This work investigates the first demonstration of nonlinear regime two-line atomic fluorescence (NTLAF) thermometry in laminar non-premixed flames. The results show the expediency of the technique in the study of the reaction zone and reveals interesting findings about the indium atomization process. Indium fluorescence is observed to be strongest at the flame-front, where the temperature exceeds 1000 K. The uncertainty in the deduced temperature measurement is approximately 6%. The temperature profile across the reaction zone shows good agreement with laminar flame calculations. The advantages and inherent limitations of the technique are discussed.

Development of temperature imaging using two-line atomic fluorescence.

This work aims to advance understanding of the coupling between temperature and soot. The ability to image temperature using the two-line atomic fluorescence (TLAF) technique is demonstrated. Previous TLAF theory is extended from linear excitation into the nonlinear fluence regime. Nonlinear regime two-line atomic fluorescence (NTLAF) provides superior signal and reduces single-shot uncertainty from 250 K for conventional TLAF down to 100 K. NTLAF is shown to resolve the temperature profile across the stoichiometric envelope for hydrogen, ethylene, and natural gas flames, with deviation from thermocouple measurements not exceeding 100 K, and typically ≲30 K. Measurements in flames containing soot demonstrate good capacity of NTLAF to exclude interferences that hamper most two-dimensional thermometry techniques.

Corrections to facilitate planar imaging of particle concentration in particle-laden flows using Mie scattering. Part 2: diverging laser sheets.

Part 1 describes a model to account for the effect of particles on laser sheet attenuation in flows where particles are heterogeneously distributed and where particles are small compared with the imaged volume. Here we extend the model to account for the effect of a strongly diverging light sheet, which is desirable when investigating many turbulent flows, e.g., in two-phase combustion problems. A calibration constant, C(kappa), is derived to account for the attenuation of the incident laser sheet due to extinction of the laser beam through a seeded medium. This is shown to be effective in correcting both the effect of in-plane laser sheet attenuation and out-of-plane signal trapping due to particles in a jet flow heavily seeded with 5 g/s of 25-40 microm spherical particles. In the uncorrected case, attenuation causes up to 15% error in the mean concentration and 35% error on the rms fluctuations. Selecting an appropriate C(kappa) was found to remove the error in the mean concentration and reduce error on the rms fluctuation by half. Methods to estimate or measure an appropriate value of C(kappa) are also presented.

Corrections to facilitate planar imaging of particle concentration in particle-laden flows using Mie scattering, part 1: collimated laser sheets.

Planar nephelometry is a laser-based technique of imaging the light scattered from particles to provide information about the local number density of these particles. In many seeded flows of practical interest, such as pulverized coal flames, particle loadings are sufficiently high for the incident laser beam to be severely attenuated. Measurements in these flows are therefore difficult, and limited data are available under these conditions. Laser attenuation experiments were conducted in suspensions of spherical particles in water at various concentrations. This is used to formulate a calibration for the effects of diffuse scattering and laser sheet extinction. A model for the distribution of light through a heavily seeded, light-scattering medium is also developed and is compared with experimental results. It is demonstrated that the scattered signal may be considered proportional to the local particle concentration multiplied by the incident laser power. The incident laser power varies as a function of the attenuation by obscurement. This correction for planar nephelometry images thus extends the technique to provide pseudoquantitative data for instantaneous particle concentration measurements.